Control method and controller for ac motor

ABSTRACT

The present invention provides a control method and a controller for an AC motor in which the AC motor can be continuously operated without using a device for returning a regenerative resistance or a regenerative energy to a power source and without raising the voltage of a main circuit to an over-voltage level.  
     The control method and the controller for the AC motor includes a power converter ( 1 ) for outputting electric power to the AC motor ( 5 ); a torque limiting unit ( 6 ) for limiting a torque command relative to an arbitrary torque command; a voltage command calculating circuit ( 7 ) for calculating a voltage command on the basis of the torque command limited by the torque limiting unit ( 6 ) to control the electric power of the power converter ( 1 ) and a switching pattern generating circuit ( 8 ). The main circuit voltage of the power converter ( 1 ) is detected by a voltage detector ( 9 ). The torque limit value of a regenerative side is calculated in accordance with the level of the detected value in a regenerative side torque limit value calculating circuit ( 10 ) and set as the limit value of the torque limiting unit ( 6 ).

TECHNICAL FIELD

The present invention relates to a control method and a controller foran AC motor that suppresses the rise of the voltage of a main circuit bya regenerative energy and continuously operates the AC motor in a powerconverter that does not include a device for returning a regenerativeresistance or the regenerative energy to a power source.

RELATED ART

In driving an induction motor using an inverter, when a speed commandthat is externally inputted to the inverted changes, a speed at whichthe induction motor is driven is ordinarily linearly changed for apreset accelerating time or a decelerating time. In such a linearacceleration and deceleration, when the value of a load is constant andthere is only an inertial load such as an inertia, torque necessary forthe acceleration and deceleration may have a prescribed value without aspecial problem. However, actually, the magnitude of the load driven bythe inverter may frequently change or the load may not be completelycomposed only of the inertial load. When the magnitudes of the loadsvary or other loads than the inertial load are to be accelerated ordecelerated by a linear speed change, it unnecessarily takes long timeto accelerate or decelerate the loads. Thus, productivity isdeteriorated or an output current value or the voltage value of a maincircuit becomes excessively large, so that the inverter may sometimesstop for the purpose of protection.

To perform an optimum acceleration or deceleration corresponding to thekind or the magnitude of the load, for instance, JP-A-Hei08-172796(Patent Document 1) disclosed a method in which the output currentinformation of a power converter and the voltage information of a maincircuit and an output speed command at a current time are previouslyprocessed in the power converter that does not include a device forreturning a regenerative resistance or a regenerative energy to a powersource and an acceleration and deceleration rate is formed from theoutput of a neural network to perform an operation without raising thevoltage of the main circuit.

Further, JP-A-Hei08-172797 (Patent Document 2) discloses an inverterdevice for rotating a motor at an arbitrary speed that includes a loaddata setting unit for setting information related to a load, a load datarecording unit for recording load data set by the load data settingunit, an acceleration and deceleration pattern forming unit for formingan acceleration and deceleration pattern upon accelerating anddecelerating the motor in accordance with the load data recorded by theload data recording unit, an acceleration and deceleration patternrecording unit for recording the acceleration and deceleration pattern,and an output speed control unit for controlling the driving speed ofthe motor from the acceleration and deceleration pattern recorded in theacceleration and deceleration pattern recording unit and an externallyapplied speed command.

Further, JP-A-Hei10-257788 (Patent Document 3) discloses a controlmethod for a motor by an inverter in which the voltage of a directcurrent bus of the inverter is detected and the limit value of a torquelimiter in a regenerative side is reduced in accordance with the rise ofthe voltage of the direct current bus of the inverter upon decelerationin the control method for the motor that includes a power converterincluding the inverter, a speed controller and a control part includingthe torque limiter to prevent the over-voltage of the direct current busdue to the rise of the voltage of the direct current bus with goodresponse characteristics and without a complicated calculation.

However, in the usual methods disclosed in the Patent Documents 1 to 3,the rise of the voltage of the main circuit (the voltage of the directcurrent bus) can be suppressed upon sudden acceleration anddeceleration, however, an effect cannot be achieved in a use in whichthe speed of an AC motor is increased by the regenerative energy of aload like a machine such as a press.

Ordinarily, since torque outputted by an AC motor is limited by any of amachine to which the AC motor is connected, the AC motor or the powerconverter relative to an arbitrary torque command, the torque limitvalue of a motor side and the torque limit value of the regenerativeside are previously set. Then, the AC motor is controlled within thetorque limit.

However, when the AC motor is suddenly accelerated or decelerated or ina use in which a speed is increased by a gravity load or a load machine,the AC motor generates a regenerative torque and the regenerative energythereof returns to the AC motor to raise the voltage of the maincircuit. Accordingly, the voltage of the main circuit needs to beprevented from rising higher than an arbitrarily set level. Generally, adevice is used in which a resistance is connected in parallel with asmoothing capacitor of the main circuit to return the regenerativeresistance or the regenerative energy consuming a power by theresistance to the power source.

However, when the device for returning the regenerative resistance orthe regenerative energy to the power source, a structure undesirablybecomes complicated and a cost is disadvantageously increased.

DISCLOSURE OF THE INVENTION

Thus, it is an object of the present invention to provide a controlmethod and a controller for an AC motor in which the AC motor can becontinuously operated without using a device for returning aregenerative resistance or a regenerative energy to a power source andwithout raising the voltage of a main circuit to an over-voltage level.

In order to achieve the above-described object, the invention defined inclaim 1 provides a control method for an AC motor including; a powerconverter for outputting electric power to the AC motor; a torquelimiting unit for limiting a torque command to an arbitrary torquecommand; and a unit for calculating a voltage command on the basis ofthe torque command limited by the torque limiting unit to control theelectric power of the power converter, the control method including thesteps of: calculating the torque limit value of a regenerative side inaccordance with the level of the detected value of the voltage of a maincircuit of the power converter, and setting the calculated torque limitvalue as a limit value of the torque limit unit.

Further, the invention defined in claim 2 provides a controller for anAC motor including: a power converter for outputting electric power tothe AC motor; a torque limiting unit for limiting a torque command to anarbitrary torque command; a unit for calculating a voltage command onthe basis of the torque command limited by the torque limiting unit tocontrol the electric power of the power converter; and a regenerativeside torque limit value calculating unit for calculating the torquelimit value of a regenerative side in accordance with the level of thedetected value of the voltage of a main circuit of the power converterand setting the calculated torque limit value to the torque limit unit.

In the invention defined in claims 1 and 2, the voltage of the maincircuit is monitored and the torque limit value of the regenerative sideis controlled in accordance with the level of the voltage of the maincircuit so that the regenerative energy returning to the power converteris suppressed. Accordingly, even in the power converter having no devicefor returning the regenerative resistance or the regenerative energy tothe power source, the voltage of the main circuit does not rise to anover-voltage level and the AC motor can be continuously operated.

As described above, according to the control method and the controllerfor the AC motor of the present invention, the voltage of the maincircuit is monitored and the torque limit value of the regenerative sideis controlled in accordance with the level of the voltage of the maincircuit so that the regenerative energy returning to the power converteris suppressed. Accordingly, even in the power converter having no devicefor returning the regenerative resistance or the regenerative energy tothe power source, the voltage of the main circuit does not rise to anover-voltage level and the AC motor can be continuously operated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the structure of an embodiment of acontroller for an AC motor in the present invention.

FIG. 2 is an explanatory view showing a relation between the voltage ofa main circuit and a torque limit value of a regenerative side in thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, an embodiment of the present invention will be described byreferring to the drawings. FIG. 1 is a block diagram showing thestructure of an embodiment of a controller for an AC motor in thepresent invention.

The controller for the motor in this embodiment includes a powerconverter 1 for outputting electric power to an AC motor 5, the AC motor5 driven by the power converter 1, a torque limit circuit 6 for limitinga torque command, a voltage command calculating circuit 7 forcalculating a voltage command on the basis of the limited torquecommand, a switching pattern generating circuit 8 for forming aswitching pattern supplied to an inverter part 4 in the power converter1 on the basis of the voltage command, a voltage detecting circuit 9 fordetecting the voltage of a main circuit of the power converter 1 and aregenerative side torque limit value calculating circuit 10 forcalculating a regenerative side torque limit value on the basis of thedetected voltage of the main circuit.

Further specifically described, the power converter 1 includes aconverter part 2 for converting three-phase alternating current to DCvoltage by a power element, a smoothing capacitor 3 for smoothing theconverted voltage and the inverter part 4 for converting the DC voltageto alternating current having an arbitrary frequency and voltage by aPWM control system. The power converter 1 supplies the electric power tothe AC motor 5.

The torque limit circuit 6 limits the torque command so as to be locatedbetween a previously set torque limit value of a motor side and thetorque limit value of a regenerative side relative to an arbitrarytorque command.

The voltage command calculating circuit 7 calculates and outputs thevoltage command so that a torque is outputted on the basis of the torquecommand outputted from the torque limit circuit 6.

The switching pattern generating circuit 8 determines the switchingpattern of the power converter 1 from the output of the voltage commandcalculating circuit 7.

The voltage detecting circuit 9 detects a main circuit voltage V_(dc) asthe voltage of the smoothing capacitor 3.

The regenerative side torque limit value calculating circuit 10 is acircuit for calculating the regenerative side torque limit value T_(GL)from the main circuit voltage V_(dc) as the output value of the voltagedetecting circuit 9 and setting the regenerative side torque limit valueto the torque limit circuit 6.

Ordinarily, since the torque outputted by the AC motor 5 is limited byany of a machine to which the AC motor 5 is connected, the AC motor 5 orthe power converter 1 relative to the arbitrary torque command, a torquelimit value T_(L0) of a motor side and a torque limit value T_(G0) of aregenerative side are previously set. Then, the AC motor 6 is controlledwithin the torque limits.

When the AC motor 5 is suddenly accelerated or decelerated or when aspeed is increased by a gravity load or a load machine, the AC motor 5generates a regenerative torque and the regenerative energy thereofreturns to the power converter 1 to raise the voltage of the maincircuit. Accordingly, the voltage of the main circuit needs to beprevented from rising higher than an arbitrarily set level.

In this embodiment, to suppress the rise of the main circuit voltageV_(dc), the torque limit value of the regenerative side is restricted inaccordance with the level of the main circuit voltage V_(dc) so that theregenerative torque is not generated and the regenerative energy is notreturned to the power converter 1.

Thus, the speed control accuracy of the AC motor is consequentlytemporarily deteriorated. However, a merit resides in a case in which aspeed accuracy is not taken preferentially and an importance is attachedto a fact that a device is not used for returning the regenerativeresistance or the regenerative energy to a power source.

Specifically, as an example of a method for restricting the torque limitof the regenerative side, a relation between the main circuit voltageand the torque limit value of the regenerative side as shown in FIG. 2is defined to control the torque limit value of the regenerative side inaccordance with the level of the main circuit voltage. As the torquelimit of the motor side, the previously set T_(L0) is the limit valueirrespective of the magnitude or the level of the main circuit voltageV_(dc). On the other hand, as the torque limit value of the regenerativeside, when the main circuit voltage V_(dc) is lower than V_(OVL), thetorque limit value is the previously set torque limit value T_(G0) ofthe regenerative side. When the main circuit voltage is not lower than aV_(OVH) level, the torque limit value is set to 0 so that theregenerative torque is not generated. Further, when the main circuitvoltage is not lower than V_(OVL) and lower than V_(OVH), the torquelimit value of the regenerative side is proportional to the main circuitvoltage V_(dc) and is lowered to 0 from an arbitrarily set torque limitvalue T_(GL1). Since the torque limit value is set as described above,as the main circuit voltage rises higher than V_(OVL), the torque limitvalue of the regenerative side is more restricted. Accordingly, as themain circuit voltage rises higher, the main circuit voltage hardly risesmore. Thus, an over-voltage is avoided and an operation can becontinued.

When the present invention is actually applied at the time of a suddendeceleration command, if the main circuit voltage is not higher than theV_(OVL) level during the sudden deceleration, the speed is reduced at acommanded deceleration rate. However, when the main circuit voltage isnot lower than the V_(OVL), as the main circuit voltage rises higher,the torque limit of the regenerative side is more restricted.Accordingly, the deceleration rate is gradually moderated and the speedis reduced at a regenerative level that can be absorbed by the powerconverter. Thus, the speed can be smoothly reduced.

Now, a case in which the speed of the AC motor is raised from a loadside will be described below. The speed is controlled so that theregenerative torque is outputted and the speed accuracy is maintaineduntil the main circuit voltage rises to the V_(OVL) level. However, whenthe main circuit voltage reaches the V_(OVL) level or higher, the speedof the AC motor tries to be increased the more, because the torque limitvalue of the regenerative side is restricted so that the main circuitvoltage does not rises more. Here, the torque limit value of theregenerative side is restricted in accordance with the level of the maincircuit voltage V_(dc) so as not to generate the regenerative torque.Thus, the main circuit voltage does not rise more and the operation ofthe AC motor can be smoothly carried out. When the main circuit voltagefalls, the torque limit value of the regenerative side is immediatelywidened. Thus, the regenerated torque is generated so that the speed canbe made to correspond to each other. As described above, when the torquelimit value of the regenerative side is controlled in accordance withthe level of the main circuit voltage, if the main circuit voltagerises, the speed accuracy is deteriorated, however, the smooth operationof the AC motor can be continuously carried out without generating theover-voltage, in the power converter having no device for returning theregenerative resistance or the regenerative energy to the power source.

In this embodiment, the relation between the main circuit voltage andthe torque limit value of the regenerative side is shown in FIG. 2.However, the arbitrarily set torque limit value T_(GL1) may be made tocorrespond to the torque limit value T_(GL0) of the regenerative side.Further, not only the torque limit value is proportional to the maincircuit voltage, but also any of functions that when the main circuitvalue rises, the torque limit value of the regenerative side comes nearto 0 may be utilized. Further, a method is desired in which a next maincircuit voltage is anticipated from the relation between the inertia ofa machine or the capacity of the smoothing capacitor and theregenerative torque to restrict the torque limit value of theregenerative side.

INDUSTRIAL APPLICABILITY

The present invention relates to a control method and a controller foran AC motor that suppress the rise of a main circuit voltage by aregenerative energy and continuously operate the AC motor (an inductionmotor, a synchronous motor) in a power converter having no device forreturning a regenerative resistance or the regenerative energy to apower source.

1. A control method for an AC motor including; a power converter foroutputting electric power to the AC motor; a torque limiting unit forlimiting a torque command to an arbitrary torque command; and a unit forcalculating a voltage command on the basis of the torque command limitedby the torque limiting unit to control the electric power of the powerconverter, the control method comprising the steps of: calculating thetorque limit value of a regenerative side in accordance with the levelof the detected value of the voltage of a main circuit of the powerconverter, and setting the calculated torque limit value as a limitvalue of the torque limit unit.
 2. A controller for an AC motorcomprising: a power converter for outputting electric power to the ACmotor; a torque limiting unit for limiting a torque command to anarbitrary torque command; a unit for calculating a voltage command onthe basis of the torque command limited by the torque limiting unit tocontrol the electric power of the power converter; and a regenerativeside torque limit value calculating unit for calculating the torquelimit value of a regenerative side in accordance with the level of thedetected value of the voltage of a main circuit of the power converterand setting the calculated torque limit value to the torque limit unit.